PrevaylerModel change listener

[rnentjes]

Create a way to easily listen to changes on a certain model type. This way it becomes feasible to create and maintain indexes.

At the moment you can add any subtype of PrevaylerModel to be stored in one call, because of that we don't have a single entry point to maintain indexes and such (this will be worse with transactions).

A listener which could be added to a Dao (or just methods in the dao) could be a solution for this. Called from the transaction store command objects for example (or at commit time with transactions).